1. Field of the Invention
The present invention relates to a machine tool suitable for a machining on a reference plane side of a work piece that has the reference plane on one surface and has no reference plane on the other surface (a surface on the opposite side)
2. Related Art
A transmission case has right and left half pieces. By combining the right and left half pieces, the transmission case becomes a product. Alignment surfaces between the right and left half pieces are used as reference planes for machining the respective right and left half pieces. Accordingly, each of half pieces has the reference plane on one surface and has no reference plane on the other surface (a surface on the opposite side). When a work piece having a reference plane on only one surface and no reference plane on the other surface is worked, the reference plane must be positioned precisely with respect to a machining axis.
Normally, as a mechanism relatively positioning a tool head and a fitting jig for supporting the workpiece, a positioning system for fitting a guide bar into a guide post is known. According to such system, there is a limit to a positioning precision because a clearance must be provided between the guide bar and the guide post. Also, there exists such a problem that a vibration caused in the work piece during the machining operation is readily transmitted to the tool.
In Patent Document 1, a horizontal-type machine tool constructed to eliminate a clearance between both members is disclosed. A guide bar movable back and forth and a linear guiding means for guiding the guide bar by pushing an outer peripheral surface of the guide bar are provided to a tool head side. A fastening means for holding/fixing the outer peripheral surface of the guide bar when compressed by a fluid pressure is provided to a fitting jig side, so that the clearance can be eliminated.
In Patent Document 2, a vertical-type machine tool in which a plurality of tool heads are vertically and horizontally moved by an inverting action of a rotation frame and a shifting action of a shift frame is disclosed. In the vertical-type machine tool, instead of using the relatively positioning system using the guide bar and the guide post, a system in which a tool head side is constructed as a dynamic structure and a fitting jig side is caused to follow the tool head side is used.
In Patent Document 3, a gimbal mechanism is disclosed as a mechanism for tiltably supporting a member with respect to an axis. A general structure of the gimbal mechanism has a convex spherical receiving portion provided on one member and a concave spherical receiving portion provided on the other member and silidably contact with the convex spherical receiving portion.
[Patent Document 1] JP-B2-2756406
[Patent Document 2] JP-B-63-65444
[Patent Document 3] JP-A-2002-210650
In the mechanism such as Patent Document 1 in which the guide bar on the tool head side is fixed by the fastening means of the fitting jig, the machine structure becomes complicated, a weight becomes heavy, and the relative positioning between both members is set up fixedly. Therefore, the vibration of the work piece is readily transmitted to the tool head side, and thus a burden is imposed on the edge, the gear, and the like on the tool head side.
Also, according to Patent Document 2, by structuring the tool head side as the dynamic structure, a burden imposed on the edge, the gear, and the like on the tool head side can be lessened. However, there is a limit to a machining precision.
Although Patent Document 3 discloses the gimbal mechanism, Patent Document 3 does not disclose or suggest a way of using the gimbal mechanism for simultaneously machining the pair of work pieces.